Replace dynamic with Object? in SystemChannels (#94629)

packages/flutter/lib/src/services/raw_keyboard.dart

@@ -284,7 +284,7 @@ abstract class RawKeyEvent with Diagnosticable {
 
   /// Creates a concrete [RawKeyEvent] class from a message in the form received
   /// on the [SystemChannels.keyEvent] channel.
-  factory RawKeyEvent.fromMessage(Map<String, dynamic> message) {
+  factory RawKeyEvent.fromMessage(Map<String, Object?> message) {
     String? character;
     RawKeyEventData _dataFromWeb() {
       final String? key = message['key'] as String?;
@@ -303,7 +303,7 @@ abstract class RawKeyEvent with Diagnosticable {
     if (kIsWeb) {
       data = _dataFromWeb();
     } else {
-      final String keymap = message['keymap'] as String;
+      final String keymap = message['keymap']! as String;
       switch (keymap) {
         case 'android':
           data = RawKeyEventDataAndroid(
@@ -388,7 +388,7 @@ abstract class RawKeyEvent with Diagnosticable {
           throw FlutterError('Unknown keymap for key events: $keymap');
       }
     }
-    final String type = message['type'] as String;
+    final String type = message['type']! as String;
     switch (type) {
       case 'keydown':
         return RawKeyDownEvent(data: data, character: character);

packages/flutter/lib/src/services/system_channels.dart

@@ -240,7 +240,7 @@ class SystemChannels {
   ///  * [RawKeyboard], which uses this channel to expose key data.
   ///  * [new RawKeyEvent.fromMessage], which can decode this data into the [RawKeyEvent]
   ///    subclasses mentioned above.
-  static const BasicMessageChannel<dynamic> keyEvent = BasicMessageChannel<dynamic>(
+  static const BasicMessageChannel<Object?> keyEvent = BasicMessageChannel<Object?>(
       'flutter/keyevent',
       JSONMessageCodec(),
   );
@@ -271,7 +271,7 @@ class SystemChannels {
   ///    applications to release caches to free up more memory. See
   ///    [WidgetsBindingObserver.didHaveMemoryPressure], which triggers whenever
   ///    a message is received on this channel.
-  static const BasicMessageChannel<dynamic> system = BasicMessageChannel<dynamic>(
+  static const BasicMessageChannel<Object?> system = BasicMessageChannel<Object?>(
       'flutter/system',
       JSONMessageCodec(),
   );
@@ -283,7 +283,7 @@ class SystemChannels {
   ///  * [SemanticsEvent] and its subclasses for a list of valid accessibility
   ///    events that can be sent over this channel.
   ///  * [SemanticsNode.sendEvent], which uses this channel to dispatch events.
-  static const BasicMessageChannel<dynamic> accessibility = BasicMessageChannel<dynamic>(
+  static const BasicMessageChannel<Object?> accessibility = BasicMessageChannel<Object?>(
     'flutter/accessibility',
     StandardMessageCodec(),
   );
@@ -311,7 +311,7 @@ class SystemChannels {
 
   /// A [MethodChannel] for configuring mouse cursors.
   ///
-  /// All outgoing methods defined for this channel uses a `Map<String, dynamic>`
+  /// All outgoing methods defined for this channel uses a `Map<String, Object?>`
   /// to contain multiple parameters, including the following methods (invoked
   /// using [OptionalMethodChannel.invokeMethod]):
   ///

packages/flutter_test/lib/src/event_simulation.dart

@@ -688,8 +688,8 @@ class KeyEventSimulator {
             result.complete(false);
             return;
           }
-          final Map<String, dynamic> decoded = SystemChannels.keyEvent.codec.decodeMessage(data) as Map<String, dynamic>;
-          result.complete(decoded['handled'] as bool);
+          final Map<String, Object?> decoded = SystemChannels.keyEvent.codec.decodeMessage(data)! as Map<String, dynamic>;
+          result.complete(decoded['handled']! as bool);
         }
       );
       return result.future;